intel_iommu: add present bit check for pasid table entries

The present bit check for pasid entry (pe) and pasid directory
entry (pdire) were missed in previous commits as fpd bit check
doesn't require present bit as "Set". This patch adds the present
bit check for callers which wants to get a valid pe/pdire.

Cc: qemu-stable@nongnu.org
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Yi Sun <yi.y.sun@linux.intel.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
Message-Id: <1578058086-4288-3-git-send-email-yi.l.liu@intel.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>